The goal of this proposal is to gain insights into how HIV-1 Tat mediates its effects on host cell gene expression. Tat is among the first genes expressed during HIV-1 infection and is essential for viral gene expression and virus production. Tat increases HIV-1 gene expression by functioning as an elongation factor and interacting with TAR, a RNA sequence present at the beginning of the HIV viral transcripts, and with the host cell factors CDK9 and cyclin T1. The interaction of Tat with these and other host cell transcriptional regulators might be expected to affect host cell gene expression. Indeed, there is considerable evidence that Tat can affect the physiology of T lymphocytes, neurons and antigen presenting cells. Recent studies from our laboratory have shown that the gene expression programs of dendritic cells and macrophages are modified by pathogen exposure and that these modifications can provide important clues to pathogenesis. We have shown that dendritic cell reprogramming by HIV-1 can create conditions that favor virus spread, and that the effect is mediated by the HIV-1 transactivator Tat. This proposal is designed to improve our understanding of the means by which Tat expression causes changes in host cell gene expression in immune cells targeted by HIV-1. To accomplish this, the specific aims of the proposal are 1) To develop a library of adenovirus and retroviral vectors expressing multiple wild type and mutated Tat proteins. 2) To determine the effects of two wild type alleles of Tat on host cell gene expression in immune cells targeted by HIV-1 infection. 3) To map the domain of Tat that is responsible for modifying the host cell gene expression program. 4) To investigate the genome-scale location of the transcription factors STAT1 and IRF7 during Tat expression, as these factors are upregulated by HIV-1 infection and Tat expression. 5) To investigate the mechanism by which Tat reprograms host cell gene expression using genome-scale location analysis experiments designed to detect DNA and RNA binding.